1. Field of the Invention
The present invention relates to a method for multiplexing of digital data over a single communication channel. Specifically, the invention pertains to simultaneous transmission of multiple digital inputs over a single channel by means of a multiplexing technique herein referred to as Pulse-Duration-Multiplexing, or PDM.
2. The Prior Art
In telecommunication, and data transmission in general, transmission lines represent substantial capital investments. Many years ago, carrier telephony was first introduced to handle four telephone conversations simultaneously over a single pair of open wires. The principle used in this carrier telephony is known as Frequency Division Multiplexing, FDM. It is still widely used for multiplexing of hundreds and even thousands of voice telephone channels over a microwave or coax-cable system.
Another multiplexing method, called Time-Division Multiplexing, TDM, works on the principle that sampled data from different input signal lines are sent out as pulses in proper time sequence. In view of rapid advances in digital technology, there is a growing tendency favoring TDM in future developments.
However, while these multiplexing equipments can provide communication channels more economically than individual physical wire channels over long distances, they are not inexpensive. Furthermore, these FDM and TDM equipments are designed and manufactured mainly for use with telecommunication common carrier facilities. Thus, a private user looking for some kind of digital multiplexing equipment for specialized applications is often at a loss to find one that suits his purpose.
Both FDM and TDM equipment are designed for highly efficient utilization of the transmission band of the wires, cables, or other mediums. However, there are instances where the frequency band width far exceeds the actual requirement, and thus frequency efficiency is of less importance. For example, users of optical fiber do not often fully utilize the band width capability.
Accordingly, the telecommunications industry is in need of a multiplexing technique which can be implemented with relatively simple circuitry, which is flexible in design and application, and which is economical to use. Moreover, the multiplexing system must be transparent to data, that is, it should not pose any requirement on composition or format of the data signals except the speed limit on baud rate.